A significant prior patent is my own U.S. Pat. No. 4,846,149, issued Nov. 7, 1989, and entitled xe2x80x9cFluid Heater Using Pulsating Combustor.xe2x80x9d
Pulsating combustion has been studied since the early part of the century, and many different types of linear pulse burners, incorporating both flap valve and aerodynamic types of fuel inlets have been constructed.
Studies I have carried out relating to the pulsating blade combustor that is set forth in my U.S. Pat. No. 4,846,149 identified above, have shown that it is advantageous to achieve a resonance match between the fuel intake pipe, and the combustor itself. Generally, the concept of resonance refers to a condition in which a vibrating system responds with maximum amplitude to an alternating driving force. This condition exits when the frequency of the driving force coincides with the natural undamped oscillatory frequency of the system.
Thus, a pulse burner, operating in the resonating mode, provides the greatest potential for:
(a) a maximum amplitude pressure wave;
(b) maximum heat flux potential;
(c) maximum potential for complete combustion.
Resonance matching has shown itself to be particularly advantageous in the utilization of higher frequencies, about which a brief discussion is appropriate.
As mentioned above, an advantage of higher frequencies in commercial pulse combustors lies in the ability to control the burner noise due to the shorter sound wave-length. This means that a smaller resonant cavity is necessary in the exhaust duct to control the inherent operating sound of the combustor. An additional advantage arises in the suppression of NOx which is also due to the shorter pulse duration that interferes with the kinetics of NOx formation. Until recently, however, tubular high frequency devices ( greater than 350 Hz) were a laboratory curiosity only, and were not commercially viable due to their inherent low capacity. High efficiency pulsating combustors are presently on the market but are characterized by a low operating frequency of around 50 Hz. This is necessary in a tubular unit so that the capacity and surface area for heat transfer is large enough to provide a practical size of domestic burner.
The pulse blade combustor which is set forth in my above-identified U.S. Pat. No. 4,846,149 operates in the same linear mode as a tube pulse burner, but burns on a flat rather than a circular flame front. The novelty of that approach is apparent in view of the fact that it was hitherto believed by researchers in the field that the viscous drag over a vastly increased heat transfer area would inhibit the combustion. This was found not to be the case, and I was able to successfully construct an operating pulse blade combustor incorporating aerodynamic valving of natural gas, the unit having a width of approximately 30 cm and a length of approximately 36 cm. The operating frequency was 441 Hz and the gas consumption was nominally 100,000 BTU/Hr. This unit is adapted for incorporation into a water heater which, with some residual heat reclaimed from the exhaust gases, acts with a percentage efficiency in the high 90""s.
While the design in the U.S. Pat. No. 4,846,149 is capable of a high rate of heat transfer through the walls to a cooling medium such as water, the shape of the item in the issued U.S. patent is not conducive to compactness of size for some applications.
Other attempts to utilize a pulsating combustor to heat water have encountered problems in muffling the sound of the unit. More particularly, the prior art combustors have generally taken the shape of a xe2x80x9cbottlexe2x80x9d with an elongated neck portion (the tailpipe), and with combustion taking part in the main portion of the xe2x80x9cbottlexe2x80x9d. Unfortunately, it is found with this prior art design that the tailpipe has to be overly long in order to provide a sufficiently large heat transfer surface. With a long tailpipe, however, the frequency of the pulsating combustion is generally in the low range, typically around 50 cps. A low-pitched noise of this kind is very difficult to damp out, and as a result water heaters or boilers which utilize this pulsating combustor design tend to be very noisy.
My earlier U.S. Pat. No. 5,403,180 discloses and claims an improvement whereby the fuel intake and the combination of the combustion chamber and the tail pipe portion each have a characteristic resonant frequency depending on its dimensional characteristics. The resonant frequency of the fuel intake in the resonant frequency of the combination of the combustion chamber and the tail pipe portion are related to each other as the ratio between two whole numbers less than 6.
U.S. Pat. No. 4,968,244 to Movassaghi shows a pulsating combustor of the type including a pair of annular plate elements stacked so as to define a central space and a slot-like passage between the adjacent pair of elements. There is a combustion chamber in communication with the passage. Means is provided for feeding fuel and combustion air to the combustion chamber. There is also means for igniting a fuel-air mixture in the combustion chamber.
WO 97/20171 discloses pulsating combustors which contain two annular plate elements and a slot-like passage therebetween.
Despite the advantages attained by the combustors disclosed in the above prior U.S. patents, there remains a need for a combustor which combines compactness of construction with an adaptability to spaces of different dimensions, and which can be reliably constructed at reasonable cost.
It is an object of one aspect of this invention to provide a pulsating combustor having an annular body consisting of a plurality of individual combustor units in stacked relation, each extending about a central space. The combustor head is situated in the central space, while a housing externally of the annular body collects the combustion gases. The head may provide a single common combustion chamber for a plurality of radially extending tail-pipe spaces, or alternatively may provide, in cooperation with the structure of the annular body, a plurality of separate combustion chambers, one for each tail-pipe space.
In a variant, the flow of gases can be reversed, with a combustion head defined by a housing around the outside of the annular body and the housing means in central aperture of the annular body for collecting combustion gases and removing these from the burner.
More particularly, this invention provides a pulsating combustor comprising:
a plurality of like annular plate elements stacked in spaced vertical alignment so as to define a central round space containing the common axis of the elements and a plurality of slot-like passages, one such passage between each adjacent pair of elements;
a first housing within the round opening and a second housing externally of the elements;
one said housing defining at least one combustion chamber in communication with at least one of said slot-like passages;
first means for feeding fuel and combustion air to said at least one combustion chamber;
second means for igniting a fuel-air mixture in said at least one combustion chamber;
the other housing defining a collection chamber for combustion gases emerging from said at least one slot-like passage.